Reference to the train Mr. Hankey rides on in a South Park episode. A cute little song accompanies his arrival on the poo-choo train:

Mr. Hankey: Poo-Choo Train's layin' down its tracks with a -
Train Whistle: poo-choo
Mr. Hankey: - all the way and back!
Cartman: Poo-Choo Train is my favorite thing, spreading Christmas joy as we ride and sing!
Mr. Hankey: Christmastime wouldn't be the same without hugs and kisses and a Poo-Choo Train.

Socailly, Poo-Choo used to express the arrival or existence of somethingbad or unfortunate.

The Urban Dictionary Mug

Alien creatures that have found their way into the anime Excel Saga. When alive they resemble sort of mutant teddy bears, however a thwack in the nose will not only kill them, but they mutate to a form where their face somewhat resembles Al Jolsten's. Their special abilities are to overpower people with their deception by sheer cuteness. Their name is derived from the only sound they make when they're not being killed. (They tend to say things like "Ah, that hurt!" as they die.)